Much attention has recently been directed to polyaniline, due in large part to its excellent stability and the relatively high levels of electrical conductivity of certain of its structural forms. For example, one form of polyaniline known as "emeraldine base" or "polyaniline base" (structure (1), Y=0.5) can be protonated (doped) by dilute aqueous protonic acid such as hydrochloric acid to produce the corresponding salt (structure (2), A=Cl). This salt exhibits conductivities of about 1-5 Siemans per centimeter (S/cm) as a compressed powder pellet. ##STR1##
Polyaniline polymers can De processed into a variety of useful shaped articles such as fibers, films, and composites thereof. For many such applications, there is a positive correlation between increased molecular weight for the polyaniline employed and the performance properties of the resultant shaped article. Considerable attention has therefore been devoted to the development of polyanilines having high molecular weight. For example, M. Abe, et al., J. Chem. Soc., Chem. Commun., 1736-39 (1989), reported that polymerization of aniline at -3.degree. C. to -5.degree. C. produced polyaniline having a weight average molecular weight of 160,000 as measured by gel permeation chromatography (GPC) using a polystyrene standard and a 0.01 mol/dm.sup.3 LiBr/NMP solution as eluent. Abe, et al. observed that the molecular weight of polyaniline increases as its polymerization temperature decreases and hypothesized that the suppression of side reactions by lowering the polymerization temperature contributed to the increase in molecular weight.
Polyaniline polymers used to form shaped articles should have relatively low polydispersity, which is defined as the ratio of weight average molecular weight to number average molecular weight (i.e., M.sub.w /M.sub.n). As will be recognized, polydispersity, in conjunction with molecular weight, has a pronounced effect on properties such as melt viscosity, tensile strength, modulus, impact strength (toughness), and resistance to heat and corrosives. Lower polydispersity values generally indicate more controlled polymerization processes and higher quality polymers.
The solubility of the polyaniline is also an important processing consideration. Crosslinking a low molecular weight polyaniline to increase its molecular weight may render the polymer relatively insoluble in most known solvent systems and, hence, unusable from a manufacturing standpoint.
It would therefore be of great advantage to provide high molecular weight polyanilines of low polydispersity which are readily soluble in a variety of solvents.